The present invention relates to a filter system for purifying a gas stream, such as an air stream, for example, an exhaust air containing radioactive materials, by means of flowable or trickable contact filter material, such as, for example, activated carbon. The filter system has a capability of 99.99% rate of filtration which is highly desirable in nuclear plants. The gas stream flows through the filter material and the filter material forms a movable bed. The filter system includes a flow-through filter chamber provided with an inlet and outlet opening for the filter material between which the filter material is conducted continuously or discontinuously by gravity.
Such filter devices serve to remove as completely as possible contaminants carried along by the stream of gas or air. in areas in which radioactive substances are being handled, activated carbon, which may additionally be impregnated, has achieved great significance in this connection. The activated carbon is here poured into the filter chamber through a fill stud, and, after a certain period of dwell in the filter chamber, is removed from the filter chamber through a corresponding discharge stud and replaced by new filter material. Since a minimum degree of decontamination must be assured by a filter system, the exchange of the entire amount of filter material becomes mandatory, even when the filter material is only partially charged with contaminants. The resulting incomplete utilization of the filter material capacity is additionally adversely influenced because the filter material is not only charged with the contaminants for which the filter device is designed, but is additionally charged with foreign substances in the form of already-present impurities in the air from the environment, as well as moisture in the air.
Due to the usually considerable costs for the filter material, this process represents an economically unsatisfactory state. Moreover, when toxic or radioactive substances are involved, care has to be taken that neither the operating personnel nor the environment are endangered during operation of the system and exchange of the filter material.
German Offenlegungsschrift No. 2,239,827 and German Offenlegungsschrift No. 2,218,360 already disclose such filter systems. In these filter systems, however, a time-dependent poisoning in the direction of the air stream does not produce optimum utilization of the filter material, even if the filter bed is subdivided. Further, the full filter bed depth is not always available during exchange of the filter material so that the stream of air must be choked or turned off. If the filter material settles during operation, leaks or cavities, respectively, develop which may cause spreading of contamination.
Filter devices are known in which the filter bed is charged with the stream of gas or air according to the counter-current principle with the aim of optimum utilization of the filter material predominantly in a direction counter to gravity. These filter devices which operate according to the counter-current principle have a very limited field of use because, even with very slow linear flow-through speeds of the stream of gas or air, the filter bed will be partially loosened up and thus cannot assure a minimum degree of decontamination.